Any discussion of the background art throughout the specification should in no way be considered as an admission that such background art is prior art, nor that such background art is widely known or forms part of the common general knowledge in the field.
The visual field of humans is commonly assessed by static perimetry. In static perimetry it is common practice to sequentially present small spots of light at each of a preset collection of points distributed across the visual field. Following each presentation of a test spot subjects indicate whether or not they have seen the test light just presented while they maintain their gaze on a fixation target. Typically subjects will give a behavioural response, such as pressing a button, to indicate that they have seen the spot. Particular parts of the visual field can have their own visual ability. The goal of perimetry is thus to assess the visual ability of each part of the visual field.
Visual ability is often characterized by sensitivity or reliability for seeing the spot stimuli. Thus for these behavioural based forms of perimetry two basic methods exist: supra-threshold and threshold perimetry. In supra-threshold perimetry several presentations are made at each of a set of preset locations in the visual field, and then the frequency of correct responses is used to estimate visual ability. In threshold perimetry the intensity or contrast of the test stimuli is varied according to a strategy to determine the fraction of the starting stimulus strength, i.e. a threshold, at which a criterion minimum frequency of seeing is obtained. Other stimulus variables that are manipulated in order to determine local visual sensitivity are the number of small objects presented to a given test region, or the size of the test stimuli presented. Visual sensitivity is thus equated with the inverse of the threshold stimulus fraction, small threshold values thus equating to large sensitivities.
An alternative and more objective method of mapping the visual field is to use so-called multifocal methods. In these methods one uses an ensemble of stimuli each presented to a different visual field location. The appearance or non-appearance of stimuli at each location is modulated by temporal sequences that are mutually statistically independent. Ideally this statistical independence should be complete, i.e. the modulation sequences should be orthogonal. A variety of patents related to particular orthogonal (U.S. Pat. No. 5,539,482 to Maddess & James, the disclosure of which are wholly incorporated herein by cross-reference) and near orthogonal sequences including (U.S. Pat. No. 4,846,567 to Sutter), exist but recent analysis methods permit more general stimuli to be used as described in U.S. Pat. Nos. 6,315,414; 7,006,863 and International Patent Publication No. WO 2005/051193, all to Maddess & James, the disclosures of which are wholly incorporated herein by cross-reference.
The general idea of multifocal methods is that the temporal statistical independence of the stimuli permits many sequences to be presented concurrently, and for the estimated response to presentations at each location to be recovered from recordings of neural activity of the visual nervous system. The neural responses to the stimuli can be recorded by electrical or magnetic sensor or detectors, changes to the absorption, scattering or polarization infrared light or other electromagnetic radiation, functional magnetic resonance imaging, or responses of the pupils.
Static perimetry arose from dynamic perimetry in which handheld stimuli of fixed sizes were moved from the peripheral visual field towards its centre, i.e. the point of gaze fixation. In dynamic perimetry the subject indicates at what point the test stimulus is seen along its centrally directed trajectory. The minimum sized stimulus that can be seen at a given distance from the centre of the visual field is taken as an indication of visual sensitivity. The most influential dynamic perimetry system is the 1945 Goldmann system. The Goldmann system defined a set of standard stimulus spot sizes. These have subsequently been quite universally adopted as the standard stimulus sizes for most static perimeters. Static perimeters offer automated standardization of the test procedure and mainly for that reason have largely supplanted dynamic perimetry. The word static in the name derives from the test stimuli being flashed at fixed points, these points typically being arranged in a regular sampling grid. The two commonest systems for conducting static perimetry tests are various versions of the Humphrey Field Analyser (HFA) produced by Carl Zeiss Meditec, and the various Octopus Perimeters produced by Haag-Streit AG. As an indication of the influence of these devices perimeters manufactured by other companies often claim substantial equivalence with the HFA to obtain approval by the United States Food and Drug Administration.
The standard test stimulus for many static perimeters is the Goldman size III. Occasionally the larger size V stimulus is used. Test grids employed in the most common static perimetry tests cover the central 24 to 30 degrees of the visual field. The sample grid is a square grid of points, with a typical separation of 6 degrees and 50 or more of these test locations are examined. The axes of the test grids are oriented horizontally and vertically. Some static perimeters permit the test grid to be uniformly shrunk or expanded to have a separation of 2 degrees. The HFA is frequently regarded as the gold standard and has been the largest selling perimeter. The most commonly used HFA test, which others emulate, is the HFA 24-2 test pattern with its 6 degree separation of test points. The Goldmann size III spot has a diameter of 0.431 degrees. The standard HFA 24-2 test grid has 54 test locations so the test spots collectively cover 7.84 deg2. The area of the visual field spanned by the 24-2 pattern is 1368 deg2 (i.e. the grid of points in the 24-2 pattern defines 38 squares, each 6 degrees on a side hence 38*36 deg2=1368 deg2). Thus the test spots collectively sample only 0.573% of the tested visual field area. Most other perimeters have similarly low coverage of the visual field. Evidently there is considerable scope to miss significant details of the visual field. This problem is commonly referred to as undersampling. Undersampling was less of a problem when the same spot sizes were used in dynamic perimetry but where the spot was swept with an unbroken motion along paths across the visual field, there was the potential that no part of the retina was missed.
If two-dimensional sampling techniques are considered, it is clear that the consequences of undersampling are worse than simply missing valuable or important information. Rather, when the sampling grid is too coarse to capture rapid changes in sensitivity across the visual field, the HFA sampling scheme is capable of distorting the appearance of the measured visual field. This occurs when the sampling grid is too coarse to capture rapid changes in sensitivity across the visual field. More specifically, any sampling grid with a regular spacing of s degrees defines a critical sampling frequency, Sc, which is the highest spatial frequency the sampling array can reliably represent. Sc is sometimes called the Nyquist sampling frequency. For the 6 degree sampling spacing common in static perimeters Sc varies between 1/12 cycles per degrees (cpd) horizontally and vertically, to 1/(12*√2) diagonally. Thus, if the visual field has spatial modulations that vary faster than Sc cycles per degrees (cpd) that these will appear in the sampled field as lower spatial frequencies through a process called aliasing. Moiré patterns and the ‘jazzing’ effects of thinly striped objects viewed on television are common examples of artifacts caused by aliasing.
These distortions of the sampled image occur because the spatial frequencies that are higher than Sc, S>Sc, that occur between than NSc and (N+1)Sc (where the N are the odd integers starting with 1) will appear to have frequencies Sc-rem(S,Sc), and at frequencies rem(S,Sc) for even N and beginning with 2, rem being the remainder function. More simply frequencies in the visual field above Sc appear as some frequency lower than Sc at various phases and orientations producing spatially distorting effects. Because these higher frequencies masquerade as low frequencies these incorrectly measured frequencies are sometimes referred to as aliases, and the process as aliasing.
Anti-aliasing filters are very common in the front-end electronics of digitizing systems. That is, higher frequencies that the sampling frequency can reconstruct are removed before sampling, however, such temporal filters do not assist in the removal of any spatial aliasing. Therefore, there is a need for an improved assessment method which can overcome the effects of spatial aliasing in the test stimuli.